Heretofore, a helmet has a shell on an outer side and a shock absorbing liner disposed on the inside of the shell. As the shock absorbing liner, polystyrene (PS) foams, polypropylene (PP) foams and the like have been used but PS foams have a drawback in that they have poor restorability after releasing compression upon impact shock. Further, PP foams have a drawback of undergoing a large reduction in compression strength at high temperature.
Further, in helmets using thermoplastic resins for the shell members, it has been difficult in view of strength or the like to satisfy JIS class C standards due to the foregoing drawbacks of the shock absorbing liner. For satisfying the JIS class C standards, FRP of high strength is used as the shell material or the thickness of the liner is increased for coping with the foregoing problems.
The prior art use of FRP of high strength as the shell material or increase in the thickness of the liner, results in drawbacks in that the shape of the helmet is enlarged, the helmet is made heavy, production cost is increased or user's demand can not be satisfied in view of the design.
An object of the present invention is to provide a helmet showing less temperature dependent change of the compression strength, excellent impact absorption and restorability, excellent impact absorption upon second hitting on an identical portion and also excellent dimensional accuracy in manufacture.
Another object of the present invention is to provide a helmet having satisfactory compatibility between its shock absorbing liner and its shell and dimensional accuracy, without increasing the thickness of the shock absorbing liner and capable of easily satisfying user's demand in view of the design.
JIS class C standard will be explained here. Helmets are specifically defined in "Protective Helmets for Vehicular Users" of Japanese Industrial Standards (JIS-T 8133). Such helmets are classified in three types: A, B, C in which A is directed to a vehicle having an exhaust capacity of 125 cc or less, B is directed to two-wheeled automobiles according to road regulations in Japan and C concerns helmets for racing, as well as B. Regarding C, it is specified in (a) that an impact acceleration of 2945 m/s.sup.2 (300, 3 G) is prevented, and (b) duration time is 4 ms or less upon an impact shock acceleration of 1475 m/s.sup.2 (150, 4 G) or higher.
In the measurement of the impact acceleration, a striker for an impact absorbing test is dropped without vibrations from the height shown in FIG. 9 and impact shock transmitted by way of a safety cap when a predetermined impact point of a specimen collides against a steel anvil is measured by an acceleration gage and a measuring recording device connected therewith, to examine whether or not the value satisfies the above-mentioned numerical standard.
Measuring conditions and the like are according to FIG. 9 and the following descriptions.
While the steel anvils used in the test inspection are different depending on the type of the safety cap as shown in FIG. 9 and, in the class C safety cap, planer type steel anvils are used at two positions and semi-spherical steel anvils are used at the other two positions of the four impact points of the safety cap.
In class B and class C safety caps, the test range is defined as a portion above a head. However, a portion within 50 mm from the edge of the cap body is excluded. The impact shock points are determined at four optional positions within the test range, and the distance between each of the impact shock points is defined as 1/5 or greater for the maximum circumference of the safety cap. Further, the number of impact shocks applied on one identical impact shock point is as shown in FIG. 9. An outline of a testing device is shown in FIG. 10 to FIG. 12.